The joints of a human skeletal structure typically comprise a ball and a socket. Joints, such as the hip and shoulder, provide for universal motion, i.e, relative motion about three transverse pivot axes.
For example, a hip joint includes a femoral head which is joined to the proximal femur by a neck which is angularly disposed relative to the axis of the femur and relative to the vertical axis of the human body. A natural socket or acetabulum receives the femoral head and cooperates therewith to form a universal joint which permits relative motion about three transverse pivot axes.
Various progressive diseases, such as osteoarthritis, can bring about deterioration of the natural socket and/or the natural femoral head. When this occurs, the diseased component can be replaced or rebuilt using an appropriate prosthetic device.
For example, in total hip replacement, an acetabular cup is cemented into the acetabulum, and the natural femoral head and neck are removed. A femoral member comprising an elongated stem, a neck and a head is mounted within the proximal femur be cementing of the stem into the femur. The head is received in the cup to provide the desired universal motion. One example of this construction is shown in Chambers U.S Pat. No. 3,656,184.
In another form of total hip replacement, a femoral cup is mounted on the head of the prosthetic femoral member. One construction of this type is shown by way of example in Averill U.S. Pat. No. 3,863,273. In this construction, the head is mounted within the femoral cup for universal movement, and the femoral cup is mounted for universal movement within the acetabulum. Accordingly, as the hip joint is used, the motion may be accommodated by either or both of these interfaces. Unfortunately, a pseudocapsule may form and impede the movement of the femoral cup in the acetabulum. In this event, motion occurs primarily or entirely between the head and the femoral cup, and this can accelerate deterioration of the cartilage by uneven lubrication between the femoral cup and the natural acetabulum and the creation of local stresses in the natural acetabulum due to the lack of relative movement between the femoral cup and the natural acetabulum.
Oh application Ser. No. 340,027 filed Jan. 18, 1982, overcomes this problem by providing a hip joint prosthesis in which some of the universal motion of the joint is forced to occur in the natural socket. Because the motion is forcibly shared between the prosthesis and the natural socket, the femoral component does not become locked in the natural acetabulum, and much more even lubrication of the acetabulum occurs.
The forced motion-sharing concept is very sound. Unfortunately, the cup assembly which provides the forced motion-sharing comprises four separate components, and these components are not as easy to assemble as is desired. More specifically, in the construction of the prior application, the cup assembly includes an insert comprising three separate components which are threadedly retained within a cup or cap. It was believed that threading was necessary in order to provide the desired strong securement between the components of the insert and the cup.